In the early 1980s, the electronics industry perceived a need for a standardized 16 and 32 bit backplane that would allow a wide variety of devices from diverse manufacturers to coexist and interact within a single system. In response to this need, an international group of major electronics manufacturers began development of the VME bus standard. This standard specifies both hardware requirements, such as connector configuration and placement, and signaling requirements. Since then, the VME bus has been further developed and has become an official IEEE and IEC standard. The VME bus is currently the most popular 16/32 bit backplane bus in the United States and Europe, and is widely used in industrial control, digital testing, signal processing, and as a building block for general purpose computers. Thousands of VME bus-compatible cards are available in the marketplace.
However, the VME bus standard was perceived to be too general and unspecific for widespread use in the field of instrumentation. Therefore, a consortium of manufacturers recently developed specifications for the VME bus Extensions for Instrumentation, known in shorthand as the VXI bus.
For a complete understanding of the environment in which the system described herein operates, the reader is referred to the following publications, which are incorporated in the detailed disclosure herein by reference: The VMEbus Specification Manual, Revision C.1, published October 1985 by Micrology pbt, and available from Printex or the VMEbus International Trade Association (VITA), both of Scottsdale, Ariz.; and VMEbus Extensions for Instrumentation: VXIbus System Specification, Revision 1.2, published June 1988, which can be obtained through the VXIbus Consortium, P.0. Box 370599, San Diego, Calif. 92137.
VME cards are, broadly speaking, plug-compatible with the VXI standard, but there are significant differences between the two standards. Although the above-referenced documents describe the VME and VXI systems completely, it will be convenient here to briefly discuss some of these differences.
One significant difference is that the VXI standard provides bus conductors and conductor definitions in addition to those of the VME standard. The VME standard provides two identical connectors (referred to as P1 and P2) on the VME backplane. The VXI standard provides a third identical connector (the P3 connector) on its backplane to allow access to the bank of additional conductors. Further, many of the conductors terminating on the VME P2 connector are user-defined, while all conductors on the VXI P2 and P3 connectors are defined by the VXI standard. Therefore, when a VME card is plugged directly into a VXI backplane, the signals from the VME card ordinarily carried on the user-defined conductors of the P2 connector may be incompatible with VXI system uses of those conductors.
Another primary difference between the VME and VXI systems lies in the allocation of address space. In discussing address space herein, the lowest 65536 words of address space 0-65535 will be referred to as A16 space, since it can be addressed using 16 binary lines. Similarly, the term A24 space will be used to describe addresses from 65536 to 16,777,215, and A32 space will be used to describe addresses between 16,777,216 and 4,294,967,295.
The VME standard does not place limitations on the use of the lowest 65,536 word addresses, while the VXI standard uses this address space for a special purpose. Specifically, the A16 address space of the VXI system is divided into up to 256 groups of 64 sequential addresses, each of which contains configuration information for a single VXI device in the system. These address groups form configuration registers, which are accessed by the system controller at power-up and are also used during system operation. Thus, A16 address space is not generally available for use by VXI devices. Another limitation imposed by the VXI standard is that VXI devices may use addresses in A24 space or A32 space, but not in both. A final significant difference is that, although the VME standard treats all component cards in the same manner, the VXI standard defines two classes of cards that operate differently in the system. These card types are message-based devices and register-based devices.
It will be appreciated that the increased specificity of the VXI standard renders many VME cards incompatible with VXI systems. First, signals on VME user-defined conductors may be incompatible with VXI system signals carried on those conductors. Second, any VME card that uses substantial A16 addressing for communications registers or data storage will interfere with operation of the VXI system. A VME card that uses both A24 and A32 addresses will be incompatible with the VXI standard. A VME card will not generally include a block of configuration registers conforming to VXI requirements. Finally, VME cards do not know whether they are message-based or register-based devices according to the VXI standard. Therefore, it is difficult for a VME card to interact with connected devices in a VXI environment.
If VME cards are used with a VXI system, the system must be configured carefully around the VME card since the VME cards may use addresses that would interfere with the configuration registers. Therefore, installing VME cards directly in a VXI bus system requires careful planning and manual configuration of the entire system to prevent addressing conflicts
Since there are an abundance of cards available for VME systems, it would be useful to develop a method by which these VME cards could be used in a VXI environment without the need for manual configuration and the reduced flexibility resulting therefrom.